Method for detecting elements

ABSTRACT

A method for the detection or determination of an element in a substance comprising that element by monitoring the e.m.f. generated between the substance and a reference material, in which the reference material is a solid electrolyte comprising a  beta -alumina containing said element or a solid compound of said element separated from the substance by said electrolyte.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of my earlier application Ser. No.592,200 filed July 1, 1975, now U.S. Pat. No. 4,085,023.

The present invention relates to the detection and determination ofsmall quantities of single elements normally in the presence of othermaterials, and in particular to the detection and determination ofimpurities of specific elements in solid or molten metal or alloys.

There are numerous industrial applications where it is advantageous tobe able to measure simply and quickly the levels of such impurities. Forexample, in the field of metallurgical refining it is often desirable toestimate impurities whilst the metal is still in the molten state sothat the composition may be adjusted before it is cast.

A galvanic method for determining the sodium activity in molten metalhas been proposed using a probe based on a sodium β-alumina solidelectrolyte (Smeltzer et al, Canadian Metallurgical Quarterly Vol. 12No. 2 (1973) pages 155-158). The probe consists of a pellet of sodiumβ-alumina sealed with Kovar 7052 glass to an α-alumina tube containing amolten metallic sodium reference. This probe was used to determine theamount of sodium present in molten tin at 500° C., but failed at thistemperature after four hours due to break down of the glass seal as aresult of attack from the molten sodium reference. This failure of theprobe renders it unsuitable for use in industrial applications, such asaluminium refining, where the temperatures to be withstood are muchhigher e.g. 750°-800° C., and more particularly where the risk ofcontamination of the metal with sodium escaping from the referencecannot be tolerated, not withstanding the severe hazards incurred whensodium is heated to such elevated temperatures. An industriallyapplicable β-alumina probe has now been constructed.

According to the present invention there is provided a method for thedetection or determination of an element in a substance comprising thatelement by monitoring the e.m.f. generated between the substance and areference material, in which the reference material is a powdered solidcompound of said element separated from the substance by an electrolytein pellet form comprising a β-alumina containing said element.

The invention also includes within its scope an apparatus for carryingout the method of the invention.

Accordingly, the invention includes an apparatus for use in thedetection or determination of an element in a substance comprising saidelement by monitoring the e.m.f. generated between the substance and areference material, in which the reference material is a powdered solidcompound of said element separated from the substance by an electrolytein pellet form comprising a β-alumina containing said element.

The method and apparatus of the present invention may be utilised forthe detection or determination of small quantities of single elements ina wide range of situations. For example, the element may be present as acomponent in the vapour or solid phase. More usually, however, thesubstance comprising the element is in the liquid or molten state, andthe invention is particularly applicable to the detection ordetermination of impurities in molten metals or alloys.

The elements which may be detected or determined by the presentinvention are typically those which form β-aluminas. Examples of suchelements are lithium, sodium, potassium, rubidium, copper, silver,thallium, indium and gallium, the various β-aluminas having a structuresimilar to that of sodium β-alumina with the element in questionsubstituting for sodium.

Particular attention is paid in the present specification to thedetection and determination of sodium, especially in the context of theproduction of aluminium and aluminium alloys, but it will be appreciatedthat the invention extends to other elements and other environments.

The composition of the β-alumina of the solid electrolyte pellet of thepresent invention may vary widely, though when the element in questionis sodium the composition is typically in the range from Na₂ O. 5Al₂ O₃to Na₂ O. 11Al₂ O₃, and β-aluminas of other elements e.g. Li, Na, K, Rb,Cu, Ag, Tl, In and Ga, are believed to have similar compositions. It hasbeen found in practice, during the determination of sodium in moltenaluminium, that when the β-alumina eletrolyte contains a high proportionof the element, i.e. towards the composition given by the formula Na₂O.5Al₂ O₃, discolouration and contamination of the electrolyte may occurduring use. Thus in a preferred embodiment the electrolyte phase alsocontains a small amount of α-alumina, for instance a mixture ofα-alumina and sodium β-alumina in which the ratio of Al₂ O₃ to Na₂ O isin the range from 11:1 up to about 12:1 or more.

The reference material may be an electrically non-conducting material,though is preferably an electrically conducting material, either anelectronic or an ionic conductor. Examples of suitable referencematerials are stable salts of the elements being detected e.g.tungstates, molybdates and vanadates, and advantageously the referencematerial consists of a 2 phase material. Preferably, there is a separatereference material which comprises a β-alumina containing the elementwhich is being determined, and is usually in the form of a two-phasematerial, for instance two-phase β-alumina, i.e. a mixture of β andβ"-alumina, or a mixture of β and α-alumina.

It will be appreciated, however, that the activity of the element in theβ-alumina reference e.g. the sodium in sodium β-alumina, is dependent toa certain extent upon the oxygen potential of the atmosphere over thereference material. Hence preferably the β-alumina reference material ismaintained in an atmosphere having a fixed oxygen partial pressure, forinstance in air under normal atmospheric conditions.

Typically the apparatus is in the form of a probe consisting at least inpart of a β-alumina of the element which is being determined, andusually the e.m.f. which is monitored is generated between a pair ofelectrodes. The probe may be in any convenient form such as a bodycomprising β-alumina so shaped such that when in use the electrode whichconnects with the reference material or the β-alumina electrolyte ismaintained out of physical contact with the substance containing theelement which is being determined. A convenient form for the probe is atube closed at one end. Thus in one embodiment the probe is in the formof a tube closed at one end consisting entirely of β-alumina, in whichcase one electrode may be secured directly to the tube for instance bymeans of a clip. It will be appreciated, however, that an additionale.m.f. may be generated in such a probe if there is a temperaturegradient between the point of attachment of the electrode and the partof the probe which is contacted with the substance. Thus preferably theelectrode is secured at a point adjacent to the part of the probe whichcontacts the substance.

It has been found however that when the probe is used in very hightemperature environments such as in molten metals e.g. at temperaturesof about 700° C. or higher, the thermal shock experienced by the probecan cause it to break and fail. A design of probe which has been foundto be particularly successful in overcoming this problem is in the formof a tube of a refractory material such as silicon nitride or preferablyα-alumina, with a pellet comprising β-alumina sealing one end of thetube. A pellet having a diameter of from about 1/8" to about 1/4" orespecially about 3/16ths" has been found to be satisfactory. The pelletmay be sealed to the end of the tube by means of a sealing material.Preferably, however, the pellet is formed in situ in the end of the tubeby hot pressing of powder comprising β-alumina, for instance atpressures ranging from about 20 up to about 100 Kg/cm² and temperaturesranging from about 1,000° up to 1,500° C. Particularly good results areachieved where both the pressure and the temperature are increased bysteps or otherwise during the hot pressing, and furthermore when thepellet is subjected to a heat treatment after pressing.

In the usual arrangement when the probe is in the form of a tube sealedat one end by a pellet comprising β-alumina, there is an electrode whichmakes internal contact with the pellet. In a preferred embodiment thetube contains on top of the pellet a layer of powdered referencematerial which is either two-phase β-alumina or a mixture of β andα-alumina, and the internal electrode is pushed through the powder tomake electrical contact with the pellet.

The apparatus is usually also provided with an external electrode, whichwhen in use contacts the substance containing the element, and may takeany suitable form. Conveniently both electrodes may be in the form ofwires preferably of a metal which is sufficiently resistant to withstandthe extreme temperature of molten metals e.g. stainless steel wires.

Generally when in use the external electrode and probe are contactedwith the substance containing the element and the e.m.f. generatedbetween the internal and external electrodes is monitored. The e.m.f.may be monitored by any suitable means though preferably a highimpedance meter is employed. When the substance is a gas or a liquidsuch as a molten metal or alloy, the probe and external electrode areinserted into the gas or liquid. When the substance is a solid the probeand external electrode are contacted with the surface of the substance.In a preferred arrangement when the substance is a gas the externalelectrode contacts the β-alumina electrolyte at its external surface.

The use of the method and apparatus of the present invention leads toadvantages in particular as a result of the very rapid e.m.f. responseof the β-alumina probe. In some cases the e.m.f. response of the probemay be practically instantaneous constituting a significant advantageover conventional glass electrodes which often take several minutes toequilibrate.

The present invention is particularly applicable to the determinationand detection of small amounts of elements present in molten metals andalloys. For instance, there are many industrial processes where it isdesirable to be able to determine small amounts of sodium present inmolten metals or alloys, and some applications of the invention are asfollows:

(a) Manufacture of Tetraethyl Lead

Tetraethyl lead is usually produced by reaction of ethyl chloride with asodium/lead alloy. It has been found that an alloy of closely definedcomposition given by the formula Na.Pb is the easiest to handle andgives the highest yields of tetraethyl lead. Thus during production ofthis alloy it is desirable to be able to closely determine the sodiumcontent.

(b) Removal of Arsenic and Antimony from Non-ferrous Metals

Impurities of arsenic and antimony which remain in some non-ferrousmetals after primary reduction are usually removed by addition of sodiumwhich forms stable arsenides and antimonides with these elements.

For example, the accepted technique in the zinc industry is to add thesodium in the form of sticks and to hold them beneath the liquid surfaceuntil reaction is complete. Often oxidation of the sodium takes place.Any additional sodium, above that required for arsenic removal, reactswith zinc to form a compound NaZn₁₃ with the result that approximately50 lbs of zinc are combined with each additional pound of sodium.Additional wastage is caused when the so formed dross is removed; thisinevitably results in entrapment of zinc, which all has to bereprocessed. In addition to combining with zinc to form a compound,there is solubility of sodium in zinc and this has the unfortunateeffect of increasing the oxidation state of the zinc in all subsequentoperations involving the material. Once again precise monitoring of thesodium content is of considerable importance.

For instance, in one method a sodium sensitive probe is inserted intothe molten zinc and the e.m.f. is monitored throughout the addition ofthe sodium. When all the arsenic or antimony has reacted with thesodium, additional sodium alters the e.m.f. of the cell permitting fargreater control than is excercised at present.

(c) Modification of Al-Si Alloys

Silicon is often added to aluminium used for castings to increase itsstrength and wear resistance. However, cooling of the material must bevery fast or the structure of the silicon in the aluminium-siliconeutectic is very coarse leading to a dull brittle alloy with poormechanical properties. It has been found that addition of sodium to themetal prior to casting gives a much finer eutectic structure and amaterial with improved properties and casting behaviour. The levels ofaddition of sodium are quite critical, an excess of sodium causingover-modification which reduces the tensile values appreciably. Forexample, for an Al-7.5% Si alloy the desired sodium content formodification is between 0.006% and 0.117% by weight. Hence it isadvantageous to be able to measure the sodium content of this system,and in particular rapid determination is highly desirable as sodiumvaporises quickly at the temperature of molten aluminium and the contentis constantly changing. Thus again the present invention is particularlyapplicable in this case.

(d) Aluminium Production

When aluminium is tapped from the Hall Cell it contains up to 0.1wt%sodium and, in order to eliminate the sodium the bath of moltenaluminium is left to stand in the atmosphere allowing the sodium toevaporate. This leads to pollution of the atmosphere, energy is expendedin maintaining the molten bath and also oxides form on the surface ofthe bath and some aluminium is lost when this oxide layer is removed.Thus it is desirable to be able to measure the sodium content so thatpollution, energy loss and aluminium loss may be minimised.

An apparatus and method according to the present invention will now bedescribed by way of example with reference to the accompanying drawingsin which:

FIG. 1 is a diagrammatic representation of a probe, for the detection ordetermination of sodium, constructed in accordance with the presentinvention, and

FIG. 2 is a graph showing a typical curve of the e.m.f. of such a probewhen inserted into a sodium containing molten Al-Si alloy, LM6.

The probe shown in FIG. 1 consists of an α-alumina tube 1 sealed at oneend with a 3/16" diameter disc-shaped pellet 2 of sodium β-aluminacontaining excess α-alumina having approximately a composition in whichthe Na₂ O to Al₂ O₃ ratio is approximately 1:12. The probe is providedwith an internal reference electrode 3 in the form of a stainless steelwire which extends through a layer 4 of powdered two phase β-aluminareference material contacting the pellet 2 at its internal surface. Theapparatus is also provided with an external electrode 5 in the form of astainless steel wire. When in use the tube 1 together with the externalelectrode 5 are lowered into the substance 6, the sodium content ofwhich is to be measured, and the e.m.f. which is generated virtuallyinstantaneously between the internal 3 and external 5 electrodes ismonitored using a suitable high impedance meter 7.

In the probe as above, the β-alumina pellet 2 is formed in situ in oneend of the α-alumina tube 1 by a hot pressing technique. Sodiumaluminate (NaAl₂ O₃) and α-Al₂ O₃ powder (both about 6μ in diameter) arewell mixed and heated together in air at 1,400° C. for 12 hours, afterwhich the mixture is ground to a powder of diameter about 1μ in aball-mill. 0.2 Grams of the mixture, which consists of Na₂ O. 11Al₂ O₃with excess α-Al₂ O₃, is placed in an α-Al₂ O₃ tube 1 of internaldiameter 3/16" held in a carbon block in a suitable hot pressingapparatus provided with a carbon rod of diameter 3/16" by means of whichpressure may be applied to the powder. The powder is cold pressed at25Kg/cm² and the load is maintained whilst the apparatus is heated to atemperature of 1,150° C. for 20 minutes. The load is then increased to50Kg/cm² and the temperature maintained at 1,150° C. for 5 minutes,after which the temperature is increased to 1,400° C. for a further fiveminutes. Heating is then discontinued and the system is cooled underpressure to room temperature. Most of the carbon rod is drilled out ofthe α-Al₂ O₃ tube 1 and the remainder is burnt out using a small oxygenlance, the high temperatures reached during this burning operationhelping to harden the pellet 2. By the above procedure a hard pellet 23/16" in diameter and about 1/8" thick is produced in one end of theα-Al₂ O₃ tube 1 giving a gas-tight probe.

EXAMPLE 1

A probe and external electrode as described above are lowered into acrucible of molten aluminium-silicon alloy LM6 which has thecomposition:

    ______________________________________                                        Si               10-13%                                                       Cu               0.1% max.                                                    Mg               0.1% max.                                                    Fe               0.6% max.                                                    Mn               0.5% max.                                                    Ni               0.1% max.                                                    Zn               0.1% max.                                                    Pb               0.1% max.                                                    Sn                0.05% max.                                                  Al               Balance                                                      ______________________________________                                    

The sodium is added to the molten alloy in the form of a flux and thee.m.f. generated between the internal and external electrodes ismonitored. FIG. 2 shows a typical curve of the e.m.f. generated againsttime, the point A on the graph indicating the point at which the sodiumis added. As can be seen the presence of the sodium in the alloy causesa sharp and instantaneous initial rise in the e.m.f. which then decaysas the sodium is vaporised.

EXAMPLE 2

Similarly as in Example 1 the probe and external electrode are loweredinto a crucible of molten aluminium-silicon alloy LM6 and sodium isadded in the form of a flux. The e.m.f. generated between the internaland external electrodes is monitored and at a known voltage the alloy issand cast. A sample of the sand cast alloy is analysed for sodiumcontent by a combined vacuum distillation and atomic emissionspectrometry technique. The procedure is repeated a number of timesusing different levels of sodium addition and the results obtained aregiven in Table I.

                  Table I                                                         ______________________________________                                        E.M.F. generated between                                                      the internal and external electrodes of a                                     Na β-Al.sub.2 O.sub.3 probe immersed                                     in molten aluminum-silicon alloy LM6                                          containing various amounts of sodium                                          E.M.F.                                                                        generated        wt % of Na                                                   in volts         in alloy                                                     ______________________________________                                        1.75             0.003                                                        1.90             0.006                                                        2.1               0.0095                                                      2.3              0.012                                                        2.5              0.016                                                        ______________________________________                                    

I claim:
 1. A method for the detection or determination of an element ina sample containing said element, including the steps of:(1) providing asolid electrolyte pellet comprising β-alumina containing the element tobe detected or determined and a refractory material tube containing areference electrode and a solid powder reference material of a compoundof said element to be detected or determined which is solid at 750° C.and which is selected from the group consisting of: two phase β-alumina,a mixture of α-alumina and β-alumina, a molybdate and a vanadate, saidpellet sealing one end of said tube and contacting said referencematerial at one surface thereof; (2) contacting a sample containing saidelement to be detected or determined with another electrode and thesurface of the pellet opposite said first-mentioned surface; and (3)monitoring the e.m.f. generated between the two electrodes.
 2. Themethod as claimed in claim 1 wherein said reference material comprises aβ-alumina selected from the group consisting of lithium, sodium,potassium, rubidium, copper, silver, thallium, iridium and gallium.